nir: Drop "SSA" from NIR language

Everything is SSA now. Need something to distract myself with.

   sed -e 's/nir_ssa_def/nir_def/g' \
       -e 's/nir_ssa_undef/nir_undef/g' \
       -e 's/nir_ssa_scalar/nir_scalar/g' \
       -e 's/nir_src_rewrite_ssa/nir_src_rewrite/g' \
       -e 's/nir_gather_ssa_types/nir_gather_types/g' \
       -i $(git grep -l nir | grep -v relnotes)

   git mv src/compiler/nir/nir_gather_ssa_types.c \
          src/compiler/nir/nir_gather_types.c

   ninja -C build/ clang-format